Here's something about impression and click. I just copy-pasted them. Hope they'll help you understand about your concern:
Page Impression- according to hubpages
"What are impressions?
As it relates to online advertising revenue (and the Ad Program), an impression typically means an "ad impression", and occurs whenever an ad appears on a Hub and is presumably seen by a visitor.
With regards to your Ad Program reports, you are paid based on "page impressions," which are Hub views. Each page impression can include up to 4 individual ad impressions, depending on: the ad level setting of the Hub, the length of the Hub, and whether a given location has "time" to show an ad in case the viewer navigates away from the Hub. The CPM for an individual is determined based on the value of individual impressions based on traffic characteristics".
Please click link if you want more: http://pageimpressionandclick.blogspot.com/